Mitochondrial myopathies. A clinico-pathological study of cases with and without extra-ocular muscle involvement

Ultrastructural organization of muscle fiber types and their distribution in the rat superior rectus extraocular muscle

Extraocular muscles (EOMs) are unique as they show greater variation in anatomical and physiological properties than any other skeletal muscles. To investigate the muscle fiber types and to understand better the structure-function correlation of the extraocular muscles, the present study examined the ultrastructural characteristics of the superior rectus muscle of rat. The superior rectus muscle is organized into two layers: a central global layer of mainly large-diameter fibers and an outer C-shaped orbital layer of principally small-diameter fibers. Six morphologically distinct fiber types were identified within the superior rectus muscle. Four muscle fiber types, three single innervated fibers (SIFs) and one multiple innervated fiber (MIF), were recognized in the global layer. The single innervated fibers included red, white and intermediate fibers. They differed from one another with respect to diameter, mitochondrial size and distribution, sarcoplasmic reticulum and myofibrillar size. The orbital layer contained two distinct MIFs in addition to the red and intermediate SIFs. The orbital MIFs were categorized into low oxidative and high oxidative types according to their mitochondrial content and distribution. The highly specialized function of the superior rectus extraocular muscle is reflected in the multiplicity of its fiber types, which exhibit unique structural features. The unique ultrastructural features of the extraocular muscles and their possible relation to muscle function are discussed.

Histochemical analysis of muscle fiber types of rat superior rectus extraocular muscle

Extraocular muscles (EOMs) represent a distinctive class among mammalian skeletal muscles in exhibiting unique anatomical and physiological properties. To gain insight into the basis for the unique structural/functional diversity of EOM fiber types and to explain their high fatigue resistance, rat superior rectus muscle (SRM) was studied using histochemical techniques. Muscle fibers were typed with regard to their oxidative and glycolytic profiles generated from succinic dehydrogenase (SDH) and phosphorylase activities in combination with their morphologic characteristics. Superior rectus muscle is organized into two layers, a central global layer (GL) of mainly large diameter fibers and an outer C-shaped orbital layer (OL) of principally small diameter fibers. Five muscle fiber types were recognized within the SRM: I, II, III, IV and V. In the global layer, four muscle fiber types were identified: type I (18.25±0.96μm; 32%) showed intermediate SDH (coarse type) and high phosphorylase activity. Type II fibers (14.45±0.82μm; 22%) exhibited high SDH (fine type) and intermediate phosphorylase activity. Low SDH (granular type) and high phosphorylase activity were demonstrated by type III fibers (22.65±1.73μm; 36%). Type IV fibers (26.24±1.32μm; 10%) were recognized by their low oxidative and glycolytic reactions. In the orbital region, only three muscle fiber types were recognized; fiber types I and II were found to compose approximately two-thirds of the layer. The third orbital fiber type (type V, 10.05±0.99μm) exhibited low SDH and low phosphorylase profiles. In this paper, the functional significance of the histochemical characteristics of the EOM fiber types is discussed.

Electrodiagnostic approach to the patient with suspected myopathy

Uncovering the cause of a suspected myopathy may be challenging. However, a careful approach starts with utilizing the wealth of available information regarding the clinical and laboratory features of myopathy. Electrodiagnostic testing is then obtained (in most cases). Recognition of the pattern of EMG findings in light of the clinical and laboratory features should narrow the differential diagnosis and dictate the next steps in the evaluation. Histopathologic or molecular studies, or both may follow. Ultimately, this approach usually allows the clinician to make the correct diagnosis.

Anaesthesia and mitochondrial disease

Mitochondrial diseases, or encephalomyopathies, are an uncommon, heterogeneous group of disorders with variable clinical course and presentation. Many of these patients present for surgery, or undergo anaesthesia in the course of investigation of their illness. Unfortunately, little information exists on their management in anaesthetic texts and the literature. We report on the anaesthetic management of a paediatric patient with mitochondrial disease, and briefly discuss the pathophysiology and anaesthetic implications of these disorders.

Investigation of muscle disease

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A case of Kearns-Sayre syndrome with metaphyseal dysplasia

A case of mitochondrial encephalomyopathy (Kearns-Sayre syndrome) demonstrating bilateral symmetric metaphyseal dysplasia is reported. This represents a new association with the syndrome.

Leber's hereditary optic neuropathy and Kearns-Sayre syndrome: mitochondrial DNA mutations

Mitochondrial DNA (mt DNA) supplies extranuclear (cytoplasmic) genes which program the manufacture of 13 of the 67 peptides of the mitochondrial respiratory enzymes. The remaining 54 are coded by nuclear DNA. All human children and adults, male and female, are entirely dependent on the cytoplasm of the ovum for their complement of mt DNA; the sperm contributes none. Accordingly, mutations in the mt DNA in a mother's ova will be passed on to all her children, although not all are clinically affected. Leber's hereditary optic neuropathy is in most cases due to a mutation that leads to the replacement of guanine by adenine at position 11778 in mt DNA. This causes histidine to be inserted instead of the normal arginine at the site of the 340th amino acid in the respiratory enzyme NADH subunit 4, hence its defective function. Other point mutations in the mt DNA coding for polypeptides of the respiratory chain complex or controlling sequences coded by mt DNA have been found in other families with Leber's hereditary optic neuropathy. Mitochondrial DNA is the site of other mutations as well. For ophthalmologists, the most important of these is the rare Kearns-Sayre syndrome (pigmentary retinopathy plus muscular dystrophies, especially of the extraocular muscles). Kearns-Sayre syndrome is due to deletions in the mt DNA, which vary in size and so affect a number of different respiratory enzymes, hence the variable manifestations. Cases are usually sporadic because the disease is often so severe that affected individuals do not reproduce if they survive, but in some cases inheritance from the mother has been reported.

Infant-onset progressive myoclonus epilepsy

We report the clinical, electroencephalographic, neurophysiologic, and neuroimaging findings in eight children with infant-onset progressive myoclonus epilepsy, all of whom had muscle biopsies performed as as part of the diagnostic evaluation. Each child had myoclonic seizures, generalized tonic-clonic seizures, and neurologic regression or marked developmental delay. Four children died before 3 years of age. Electroencephalograms in seven children showed an abnormally slow background with bilateral multifocal paroxysmal discharges but no burst suppression pattern or photoparoxysmal response. Muscle biopsy specimens were submitted for histopathology and respiratory-chain enzyme studies. Nonspecific abnormalities on light microscopy or electron microscopy were found in seven samples, including increased subsarcolemmal deposits of mitochondria or morphologic mitochondrial changes, but no ragged-red fibers were seen. Respiratory-chain enzyme studies were performed on five samples and in three children (all of whom had a history of elevated lactate in serum or cerebrospinal fluid), there were low levels of rotenone-sensitive reduced nicotinamide adenine dinucleotide (NADH) cytochrome c reductase characteristic of a defect in the complex I part of the respiratory-chain pathway. This study has shown that infant-onset progressive myoclonus epilepsy can be distinguished from other myoclonic epilepsy syndromes of infancy by clinical and electrographic features. Furthermore, respiratory-chain enzyme defects are a relatively common cause of infant-onset progressive myoclonus epilepsy. The absence of ragged-red fibers on muscle histopathology does not preclude a mitochondrial enzyme abnormality.

Anaesthesia for patients with mitochondrial myopathy

The anaesthetic management for an infant with mitochondrial encephalomyopathy due to fumarase deficiency is described. Mitochondrial myopathies may produce skeletal and cardiac muscle abnormalities, central nervous system effects and metabolic problems. The solutions to the anaesthetic problems posed by these patients are discussed.

Variation in retinal changes and muscle pathology in mitochondriopathies

A variety of retinal changes that have so far not been classified under mitochondriopathies can now be included in this group, since muscle biopsy has identified ragged-red fibers with pathological mitochondriae. The ophthalmological findings in our relatively large group of 12 patients with mitochondrial myopathies are compared with the spectrum of myopathic findings. No obvious correlation exists between the severity of the pathological retinal changes and the characteristic of the myopathic alterations. In addition to fine pigmentation and depigmentation, severe dystrophic changes of the retina, pigment epithelium, and the choroid were observed. In two patients with severe chorioretinal dystrophy the correlation with generalized mitochondriopathy was not suspected prior to muscle biopsy.

Mitochondrial dysfunction in myasthenia gravis. Report of a case

The case of an 11-year-old boy with external ophthalmoparesia, tetraparesia and bilateral eyelid ptosis is reported. He was 7-years-old when first symptoms appeared. Anticholinesterasic drugs were used. He was submitted to muscle biopsy. The results of histochemistry analysis showed storage of granulous material at the subsarcolemmal region of muscle fibers by SDH. Increase in the number of mitochondria with electron dense bodies was found at electron microscopy. Anticholinesterasic drugs administration was interrupted and consequently he got worse, and bouts of dyspnea occurred. Due to this worsening anticholinesterasic agents were reintroduced together with prednisone, and he improved. Due to clinical and histological expressions we think it is possible that morphological mitochondrial alterations may occur also in myasthenia gravis.